1. Field of the Invention
The present invention is directed to a method for the manufacture of scintillator ceramic by hot pressing a powder containing at least one rare earth oxisulfide (rare earth oxisulfide powder).
2. Description of the Prior Art
Scintillator elements of the detector elements in the radiation detector system are for computed tomography (CT) are manufactured as scintillator ceramic from rare earth oxisulfides. When irradiated by x-rays, such scintillator elements produce light that is coupled out from the translucent ceramic and is converted by a photodiode into an electrical signal.
Since the intensity of the light produced by scintillator element is a measure for the intensity of the incident radiation, the output current of the photodiode represents a measure for the intensity of the radiation incident on a scintillator element.
Because of its limited by the oxygen sensitivity of the rare earth oxisulfide at elevated temperatures, the compression of the rare earth oxisulfide powder into ceramic by means of hot pressing can take place only under protective gas or in a vacuum since, temperatures of up to 1,500.degree. C. are required.
In the manufacture of rare earth oxisulfide powder and in the handling thereof, contamination by dust and abrasion particles is unavoidable. These particles are then carbonized as a result of the high temperatures during hot pressing and remain as black specks in the finished ceramic. If the scintillator element contains such imperfections, the light propagation in the scintillator element is disturbed, and such a ceramic is unsuitable as a detector for x-ray in so far as the imperfection exceeds a specific level.
In scintillator ceramic manufactured according to known methods, reject rates of a not inconsiderable level occur due to these reasons.
German OS 37 02 357 discloses supporting discoloration of a rare earth oxisulfide ceramic article pressed from rare earth oxisulfide particles by a heat treatment between 400 to 800.degree. C. in air prior to an isostatic hot pressing. The surfaces of the rare earth oxisulfide powder particles are thereby oxidized by the formation of a limited amount of rare earth oxisulfate, and this the rare earth oxisulfide is not oxidized to rare earth oxisulfate in the interior of the rare earth oxisulfide powder particles. A discoloration of rare earth oxisulfides is therefore suppressed during hot pressing by means of the formation of rare earth oxisulfate on the surfaces of the rare earth oxisulfide powder particles.